You're referring to the BIOS Setup. Your CMOS is the actual memory area itself which stores the system configuration and setup data.

To access BIOS setup, you need to press a key or a key combination at the appropriate point during the initial Boot sequence. Pressing the correct key at 1 second intervals once the memory check begins is an almost bullet-proof way of getting the timing right! :D

In many instances, and invariably if the system is an unbranded one, the <Delete> key is the one to press. But quite a few manufacturers of 'Name-brand' PCs require a different keypress. Try the alternatives you'll find in this resource list: